FIG. 1 is a diagram showing a HFC network based a digital CATV transmission system according to the related art.
A headend system 100 receives and transmits a cable signal through a plurality of cells 121, 122, 123 and a hybrid fiber coaxial (HFC) network 150.
The HFC network 150 is configured with an optical cable network 130 and a coaxial cable network 140. Optical node units 111, 112, 113 in the HFC network 150 convert an optical signal to an electric signal or convert the electric signal to the optical signal.
When a conventional digital CATV transmission system transmits a cable signal, the headend system 100 modulates a signal using a cable MODEM and converts the modulated signal to a predetermined frequency band. After converting, the headend system 100 combines the converted signal with broadcasting signals of other channels and transmits the combined signals to an optical cable network 130 through an electric-to-optical converter. The broadcasting signals of other channels are In-Band signals which are broadcasting signals in a coaxial cable usable frequency band.
The modulated and combined signals reach to the outdoor optical node units (ONU) 111, 112 and 113 through the optical cable network 130. Then, the ONUs 111, 112 and 113 converts an optical signal to an electric signal and outputs the converted signal to the coaxial cable network 140 through one or more coaxial cables. Finally, subscribers receive the cable signal through the coaxial cable network 140.
Such a conventional outdoor optical node unit (ONU) is a device for connecting an optical cable network and a coaxial cable network in a HFC network. As described above, the conventional ONU simply relays a downstream/upstream signal by converting an optical signal to an electric signal. That is, the conventional ONU simply connects the optical cable network and the coaxial cable network converting the optical signal to the electric signal or converting the electric signal to the optical signal.
However, it is ineffective in a view of a transmission efficiency since sub cells connected to one outdoor ONU receives signals identical to cable signals transmitted to other sub cells in case of transmitting of a downstream signal.
That is, the conventional outdoor ONU divides a signal as many as the number of connected coaxial cables and transmits the divided signals. When transmitting the signals, a signal transmitted to a predetermined substrate included in a predetermined sub cell is also broadcasted to other subscribers through all of coaxial cables connected to the corresponding outdoor ONU. Therefore, it is very ineffective in a view of an efficiency of using a frequency.
In order to overcome the inefficiency program and improve the transmission efficiency, a conventional cell division scheme was introduced. The conventional cell division scheme divides the cells to a plurality of sub cells. However, the conventional cell division scheme requires a mass amount of expenses in a view of economic and time.
In case of transmitting an upstream signal, the conventional outdoor ONU receives upstream signals transmitted from substrates through one or more coaxial cables in 5 to 42 MHz bands, combines the received upstream signals, and transmits the combined signals to a cable MODEM of the headend system. However, cable signals inputted through different coaxial cables are often collided. Accordingly, the transmission efficiency is degraded.
Furthermore, resources of upstream channels are reduced in proportional to the number of subscribers in each cell because subscribers in a corresponding cell share upstream frequency channels assigned within a frequency range for a conventional cable TV system.
Moreover, a conventional digital CATV transmission system ineffectively uses a network resource because the conventional digital CATV transmission system uses a band same to a coaxial cable useable frequency band although an optical cable network between the headend and the outdoor optical node unit has a higher usable frequency band than a coaxial cable network.